I am just commenting on bidding strategies while I would not like to definitely recommend something without seeing the account. The success of a bidding strategy usually depends much more on the underlying script than on its description or definition. I extensively used the Maximize Clicks bidding strategy with a budget higher than yours in 2014 and the results were epic. However, what I learned is that it's quite a bit sensitive to the lack of upper limits, therefore remember to cap properly the max CPC bid and your daily budget. The strategy needs a number of days to get things sorted out and accrue some meaningful stats; that's something you'll need to survive in terms of finance.

Negative. We used it on Google Search with the objective that users sign up for a free account with client's website. I know that this experience only tangentially relates to your query, however, thought it might be a bit interesting, with special regard to capping.

With the margins being inconsistent, you are likely not in a "can't spend enough to lose" situation like Lakatos experienced. Maximize clicks may be a little too front-end aggressive for the campaign as it currently stands.

Can't use target CPA with shopping campaigns... Sooooo - I would say try enhanced cpc first. Like Lakatos said, any time you switch to a new bidding strategy you have to be willing to take some time. Enhanced cpc would give you some more room to go up when Google thinks a conversion is more likely. Yet, the focus is still on conversions (product sales), not clicks.

Target ROAS may actually be too restrictive for you, and may have some technical issues (It can't take into account returning revenue that's past the AdWords cookie duration, for example.). Unless you've got the campaign to maximum exposure, while still being profitable, immediate ROAS may not even be your current goal.

If your mandate is to spend the $20K on exposure, and generate the most revenue you can, then what do your budget recommendations, and impression share stats look like? Have you maxed out what you have, and still come in under budget? Or, is the market much larger than the $20K budget you have?

Sorry I meant to say 20k products in a shopping campaign. My budget is unlimited as long as I'm spending money that converts. The Roas strategy has definitely been working out but I'm concerned that I will lose out on impressions for products that don't have any conversions.

Leave your premium products / product groups in the established shopping campaign, with the current settings that are working for you. Optimize for volume of sales for those premium products and (assuming the reason why they are "premium" is because they are generating a positive ROI.), spend as much as you can on that premium campaign. As long as the margins hold, you can't lose.

Unfortunately, there's only so much premium to be had. The next best, or secondary products/groups can still be profitable as part of the long tail, but - Your goals for those Groups/campaign are not the same as premium campaign(s). Maybe you only "break even" in the first/last click sense, but if you are bringing new qualified users into the funnel, that provides it's own opportunities.